As mobile devices have been increasingly developed, and the demand for such mobile devices has increased, the demand for secondary batteries has also sharply increased as an energy source for the mobile devices. Accordingly, much research into batteries satisfying various needs has been carried out.
In terms of the shape of batteries, the demand for prismatic secondary batteries or pouch-shaped secondary batteries, which are thin enough to be applied to products, such as mobile phones, is very high. In terms of the material for batteries, on the other hand, the demand for lithium secondary batteries, such as lithium ion batteries and lithium ion polymer batteries, exhibiting high energy density, discharge voltage, and output stability, is very high.
Furthermore, secondary batteries may be classified based on the construction of an electrode assembly having a positive electrode/separator/negative electrode structure. For example, the electrode assembly may be configured to have a jelly-roll (wound) type structure in which long-sheet type positive electrodes and negative electrodes are wound while separators are disposed respectively between the positive electrodes and the negative electrodes, a stacked type structure in which pluralities of positive electrodes and negative electrodes, each having a predetermined size, are sequentially stacked while separators are disposed respectively between the positive electrodes and the negative electrodes, or a stacked/folded type structure in which pluralities of positive electrodes and negative electrodes, each having a predetermined size, are sequentially stacked while separators are disposed respectively between the positive electrodes and the negative electrodes to constitute a bi-cell or a full-cell, and then bi-cells or the full-cells are wound using a separator sheet.
In the electrode assembly having the positive electrode/separator/negative electrode structure, the electrodes may be simply stacked. Alternatively, a plurality of electrodes (i.e. positive electrodes and negative electrodes) may be stacked in a state in which separators are disposed respectively between the electrodes, and then the stacked electrodes may be coupled to each other by heat/pressure. In this case, the electrodes and the separators are coupled to each other by heating and pressing the electrodes and adhesive layers applied to the separators in a state in which the electrodes face the adhesive layers. In order to improve adhesion between the electrodes and the separators, each of the separators is coated with a binder material.
In a case in which a separator coated with a binder material is used in a electrochemical cell, such as a battery, if the adhesive force between binder powder and a separator substrate, the binder may be separated from the separator substrate during electrolyte injection and degassing processes, with the result that the separator may move, whereby the external appearance of the battery cell is deteriorated while the performance of the battery cell is reduced.
One of the principal research projects for secondary batteries is to improve the safety of the secondary batteries. In general, a lithium secondary battery may explode due to high temperature and pressure in the secondary battery which may be caused by an abnormal state of the secondary battery, such as a short circuit in the secondary battery, overcharge of the secondary battery with higher than allowed current or voltage, exposure of the secondary battery to high temperature, or drop of the secondary battery or external impact applied to the secondary battery.
In addition, in a case in which a sharp needle-shaped conductor, such as a nail, having high electrical conductivity penetrates into the electrode assembly, the positive electrode and the negative electrode of the electrode assembly are electrically connected to each other by the needle-shaped conductor, with the result that current flows to the needle-shaped conductor, the resistance of which is low. At this time, the electrodes through which the needle-shaped conductor has penetrated are deformed, and high resistance heat is generated due to conducting current in a contact resistance portion between the positive electrode active material and the negative electrode active material. In a case in which the temperature in the electrode assembly exceeds a critical temperature level due to the resistance heat, the oxide structure of the positive electrode active material collapses, and therefore a thermal runaway phenomenon occurs. As a result, the electrode assembly and the secondary battery may catch fire or explode.
Furthermore, in a case in which the electrode active material or the current collector bent by the needle-shaped conductor contacts the opposite electrode that the electrode active material or the current collector faces, the thermal runaway phenomenon may be further accelerated. These problems may be more serious in a bi-cell including a plurality of electrodes and an electrode assembly including the same.
Consequently, there is a high necessity for technology that is capable of more safely and efficiently securing the safety of the secondary battery.